Single-walled Carbon Nanotubes Wrapped with Charged Polysaccharides Enhance Extracellular Electron Transfer

Microbial electrochemical systems (MESs) rely on the microbes’ ability to transfer charges from their anaerobic respiratory processes to electrodes through extracellular electron transfer (EET). To increase the generally low output signal in devices, advanced bioelectrical interfaces tend to augment this problem by attaching conducting nanoparticles, such as positively charged multiwalled carbon nanotubes (CNTs), to the base carbon electrode to electrostatically attract the negatively charged bacterial cell membrane. On the other hand, some reports point to the importance of the magnitude of the surface charge of functionalized single-walled CNTs (SWCNTs) as well as the size of functional groups for interaction with the cell membrane, rather than their polarity. To shed light on these phenomena, in this study, we prepared and characterized well-solubilized aqueous dispersions of SWCNTs functionalized by either positively or negatively charged cellulose-derivative polymers, as well as with positively charged or neutral small molecular surfactants, and tested the electrochemical performance of Shewanella oneidensis MR-1 in MESs in the presence of these functionalized SWCNTs. By simple injection into the MESs, the positively charged polymeric SWCNTs attached to the base carbon felt (CF) electrode, and as fluorescence microscopy revealed, allowed bacteria to attach to these structures. As a result, EET currents continuously increased over several days of monitoring, without bacterial growth in the electrolyte. Negatively charged polymeric SWCNTs also resulted in continuously increasing EET currents and a large number of bacteria on CF, although SWCNTs did not attach to CF. In contrast, SWCNTs functionalized by small-sized surfactants led to a decrease in both currents and the amount of bacteria in the solution, presumably due to the detachment of surfactants from SWCNTs and their detrimental interaction with cells. We expect our results will help researchers in designing materials for smart bioelectrical interfaces for low-scale microbial energy harvesting, sensing, and energy conversion applications.


Figure S1 .
Figure S1.Equivalent circuit used to model the EIS data.R.E. and W.E. are reference and working electrodes, respectively.Rohm, Rct and Rbac are the ohmic, charge transfer and bacterial resistances, respectively.Qdl and Qbac are the constant phase elements representing the electric double layer and bacteria, respectively.a7 and alpha are constants describing the corresponding non-ideal capacitances.

Figure S2 .
Figure S2.Selected Bode plots (symbols) shown also in Figure 5b and impedance fits (lines) calculated according to the circuit model shown in Figure S1 for a) MR-1 control, b) MR-1 SWCNT-HEQ and c) MR-1 SWCNT-CTAC.Spheres and red lines refer to modulus (left Y axis), while diamonds and blue lines to phase angle (right Y axis).

Figure S3 .
Figure S3.2D PL mapping images of the solubilized SWCNTs with CMCNa, CTAC and Triton X.

Figure S4 .
Figure S4.Photographic snapshots of carbon felt electrodes with functionalized SWCNTs recorded before the addition of bacteria.Scale bars are 1.2 cm.

Figure S5 .
Figure S5.Background-corrected optical density values of bulk electrolyte samples obtained at 600 nm, sampled at various instances during chronoamperometric measurements at +0.2 VAg/AgCl or immediately after the end of electrochemical experiments.For MR-1 control each of data points 4 and 5 in increasing order represents mean and standard deviation from n = 2 samples; for MR-1 SWCNT-HEQ the data point represents mean and standard deviation from n = 2 samples; for MR-1 SWCNT-CMCNa each data point represents mean and standard deviation from n = 3 samples; the rest of data points represent only one measured value.Dashed gray line denotes the value that was set as the initial OD600.

Figure S8 .
Figure S8.Combined bright field and fluorescence FITC microscopy images of carbon felt electrodes with and without functionalized SWCNTs.Green color marks the GFP mutant, wide silver rod-like structures are CF and thin black thread-like structures and areas (some of them marked by red arrows) in b are assumed to be aggregates of SWCNTs.Scale bars are 10 µm.a) I-III 'MR-1 control', b) I-III 'MR-1 SWCNT-HEQ' and c) I-III 'MR-1 SWCNT-CMCNa' images were taken after ~91, ~80, and ~100 hours of chronoamperometry, respectively.

Figure S9 .
Figure S9.Nyquist plots of bacterial reactors with (filled symbols) and without (empty symbols) S. oneidensis MR-1 bacteria (OD600 = 0.26) in the presence of SWCNTs functionalized by HEQ (red spheres), CMCNa (light blue spheres), CTAC (blue spheres), Triton X (orange spheres) and no SWCNTs present (control; black squares) in anoxygenic environment in the electrolyte M9 with lactate (40 mM). a and b represent different magnifications of the same data.Data were acquired immediately before or after the CAs shown in Figure 2. All EIS measurements were carried out at +0.2 VAg/AgCl to maintain bacterial EET.

Table S1 .
Current levels extracted from cyclic voltammetry measurements.